1. Field of the Invention
The present invention relates to a voltage regulator, and ore particularly, to a dual mode voltage regulator.
2. Description of the Related Art
Generally, for stabilizing the voltage output to an internal circuit supplied by a supply voltage, a voltage regulator is set between a supply voltage and a load.
FIG. 1 is a schematic circuit diagram showing a related circuit structure of a voltage regulator 1. The voltage regulator 1 has an OP 10 and a NMOS transistor 11. The disadvantage of the voltage regulator 1 with NMOS transistor 11 is that it requires a high supply voltage VDD. Another configuration of a voltage regulator 1′ is shown in FIG. 2. The voltage regulator 1′ has an OP 10 and a PMOS transistor 11′. Though it can be operated in low supply voltage VDD, the voltage regulator 1′ has stability problems and worse transient response. A large external compensation capacitor must be disposed at the output to determine start up delay time.
When the regulator in Multi-Media Card applications, the regulator must be operated under dual voltage (e.g. 3.3 V and 1.8 V). Thus, it is more difficult to stabilize the supply voltage without use of an external capacitor.